1. Field of the Invention
This invention pertains to methods and compositions for controlled free radical polymerization in heterogeneous aqueous media using stable free radicals that provide control under a variety of conditions and with a wide variety of monomers. This invention also pertains to the polymers, copolymers and interpolymers that can be made with such systems. Further, this invention pertains to new methods of initiating such polymerization reactions.
2. Discussion
Controlled free radical polymerizations are generally known that allow xe2x80x9cliving typexe2x80x9d polymerization to occur. These reactions typically proceed by either an atom transfer or stable radical mechanism. Other mechanisms are known, including xe2x80x9ciniferterxe2x80x9d mechanism and degenerative radical transfer mechanism. In connection with the stable radical mechanism, typically, a stable free radical is used to reversibly cap the propagating polymer chain, and there are several well-known nitroxides that may be usefully employed. See, e.g., U.S. Pat. Nos. 4,581,429, 5,322,912 and 5,401,804, each of which is incorporated herein by reference. The nitroxides disclosed. in these references have carbon atoms attached to the nitrogen (i.e., xcex1-carbon atoms) that bear alkyl or aryl moieties. One of the most common nitroxide radicals is 2,2,6,6-tetramethyl-1-piperidinoxyl radical (TEMPO) and several groups have worked to make this and related radicals a commercially viable control agent in a stable free radical polymerization scheme. See, e.g., WO 98/13392 and WO 98/07758. The drawback of using TEMPO and related stable radicals has been the limitation on the monomers that can actually be polymerized, the high temperatures required, and an inability to function well in emulsions. Styrene, substituted styrenes and copolymers including styrene have been prepared, but other desirable, commercially important monomers have not been polymerized successfully in a controlled manner to desired molecular weights. TEMPO has proven to be limited in its usefulness, while others have suggested that structures similar to TEMPO have greater usefulness. See WO 98/30601, which is incorporated herein by reference.
Alternative nitroxides were suggested by Grimaldi et al. bearing an electron withdrawing dialkoxyphosphonyl substituent and a hydrogen atom on the carbon atoms in the xcex1-position relative to the nitrogen. Grimaldi et al. xe2x80x9cSynthesis and Applications to xe2x80x98Livingxe2x80x99 Free Radical Polymerization of a New Class of Nitroxyl Radicals,xe2x80x9d Polymer Preprints, vol. 38, no. 1 (April 1997). See also WO 96/24620. By comparing the xcex1-substituted dialkoxyphosphonyl nitroxides to other nitroxides having alkyl substituents and a hydrogen atom on an xcex1-carbon, Grimaldi et al. concluded that the dialkoxyphosphonyl nitroxides provide better control and access to a greater range of monomers than TEMPO. See also EP 0891 986 A1, which is incorporated herein by reference.
Recently, Hawker et al. have shown that it is the presence of a hydrogen atom on the xcex1-carbon, rather than the dialkoxyphosphonyl group that provides a route to the controlled free radical polymerization of monomers other than styrene. Hawker et al. xe2x80x9cDevelopment of a Universal Alkoxyamine for xe2x80x98Livingxe2x80x99 Free Radical Polymerizations,xe2x80x9d J. Am. Chem. Soc., 1999, 121, 3904-3920. Also, it should be noted that many stable nitroxide radicals having a hydrogen atom on the xcex1-carbon (sometimes referred to as xcex1-hydrido nitroxides) are known. See, e.g., Janzen et al., J. Am. Chem. Soc., 1969, 91,4481-4490; Janzen et al., J. Am. Chem. Soc., 1989, 111, 2206-2070; and Janzen et al., J. Am. Chem. Soc., 1986, 108, 6858-6863.
These efforts have typically focused on polymerizations in bulk and in organic solution. However, the use of water as a dispersing medium for control free radical polymerization is commercially important for several reasons. First, water is a safe medium from an environmental viewpoint, facilitating the manufacture of consumer products (such as paints or glues). Also, water is inexpensive, providing an economical process. Moreover, the various emulsion polymerization processes can offer mechanistic and process advantages over homogeneous polymerization, in terms of reaction kinetics, molecular weight, viscosity, heat transfer, and resulting polymer structures and properties. Also, many applications of polymers directly utilize the heterogeneous aqueous polymer products of such polymerization.
Known uncontrolled or non-living aqueous heterogeneous polymerization processes are useful because they can produce a wide variety of polymer products utilizing many types of monomers in rapid economical processes at temperature below 100xc2x0 C., forming polymer particles with controlled particle sizes and polymers of high molecular weight. However, such non-living processes allow limited or no control over the polymer chain architecture, such as the formation of block copolymers or formation of narrow molecular weight distribution.
The known living or controlled polymerization processes can offer controlled chain architecture and molecular weight distribution. However, such processes have typically required very high temperatures, greater than 100xc2x0 C., utilize only limited types of monomers, produce polymer in slow, time-consuming process, and do not work well in heterogeneous aqueous media. The few heterogeneous aqueous system known require high temperature or produce polymer particles. with limited control of particle size and distribution.
Thus, a need exists for a versatile, heterogeneous water-based controlled or living free radical polymerization process, which can polymerize many different types of monomers with economically viable process conditions.
This invention is thus directed toward methods of polymerization to form heterogeneous aqueous polymer mixtures such as emulsions. The methods of this invention provide stable, living-type free radical polymerizations in an emulsion, including the ability to re-initiate polymer chains and thus prepare unique polymers and architectures, such as block copolymers, including stars, grafts, telechelics and macromonomers. It is also an object of this invention to provide a polymerization process that allows access to a wide variety of monomers that may be polymerized alone or together in emulsions. Moreover, some methods of this invention enable access to a full range of initiators, including fast and water-soluble initiators as well as slow and organic-soluble initiators that might otherwise appear to be less favorable for aqueous-based living polymerizations. It is also an object of this invention to enable the preparation of polymer emulsions with excellent control over particle size, molecular weight, polydispersity and polymer composition and architecture.
These and other benefits can be realized by an emulsion polymerization process that uses water, initiator, at least one monomer and a control agent that is an xcex1-hydrido nitroxide. The control agent can be added to the emulsion as a stable free radical, as an adduct with the initiator or as a nitrone precursor. The ratio of control agent to initiator can be in the range of from about 0.01:1 to about 4:1, but is preferably close to 1:1 to provide a commercially reasonable balance between reaction time and living character. The ratio of initiator to monomer is important to the desired molecular weight of the resultant polymer and this ratio can be adjusted to a desired target molecular weight.
Yet another aspect of this invention is a novel process for an emulsion polymerization that allows the use of slow initiators or organic soluble initiators for a living-type emulsion polymerization mixture. This novel process overcomes these problems in an emulsion system by supplying the monomer to the process in two or more stages. First, a fraction of the total monomer that is planned to be added to the polymerization reaction is first mixed with the initiator, control agent, water and surfactant. This combination is mixed and allowed to react for a predetermined period of time under predetermined polymerization conditions. The intent of this first stage is to allow nearly complete reaction of the initiator to form xe2x80x9clivingxe2x80x9d oligomers with the monomer in the system and the control agent. Second and optionally subsequent stages provide for the addition of additional monomer, which can be the same or different from the monomer used in the first stage.